Description of Research: Pellet cultures are commonly used to study chondrogenic differentiation in vitro. Our laboratory has demonstrated pellets made with chondrocytes grow in size during culture and produce cartilage matrix, but pellets made with adipose-derived mesenchymal stem cells (ASC) grow only slightly, producing little cartilage matrix. The objective of this study was to determine if differences in chondrocyte and ASC pellet growth result from differences in cell proliferation or in deposition of extracellular matrix.